The present invention relates to a method that uses a coupling eyepiece and an image capture apparatus to align an object whose image is to be recorded in order to produce an optimal quality shot.
The invention may have applications in any sectors concerned with the creation of optimal quality images, particularly in the field of astronomy and in the medical sector and more particularly for X-ray shots.
In the field of image capture, specialists in the field attempt to achieve images of the highest quality possible, in other words with satisfactory focussing of the capture system and the best possible image definition.
In order to obtain an image of excellent quality, the image plane, i.e. the plane in which the image capture apparatus lies, must be perfectly aligned with the coupling eyepiece on the one hand and with the object on the other. However, the alignment is difficult to effect as the orthogonality of the image plane must be adjusted according to the optical axis and to the focal distance.
Also, in order to create images of an object on an image plane through a welding eyepiece it is necessary to obtain optimal positioning between the object, the eyepiece and the image plane. The optimal positioning can be achieved by means of settings that are usually made between the positions of the eyepiece and the capture system (i.e. the image plane). The object is generally positioned so that it is fixed. There are two kinds of settings:
a setting, by means of rotation, of the perpendicularity of the image plane in relation to the optical axis of the eyepiece; and
a setting of the image plane position i.e. the capture apparatus, in relation to the eyepiece; this is the standard type of focus setting used in an ordinary camera. The setting is made by means of a translation movement along the optical axis, and enables the plane of the capture apparatus or the image plane to be focused on the object plane.
Generally, the object is at a variable distance from the eyepiece. The eyepiece/image plane setting can therefore be made either approximately or visually by the operator using range finding or reflex means.
Some image recording or capture apparatuses are equipped with a system for measuring the object/eyepiece distance that measures the distance between the object and the eyepiece using an ultrasound or infrared method. In this technique the eyepiece/image plane setting is made automatically.
However, in these standard apparatuses, the depth of field at the eyepiece is significant. The eyepiece/image plane setting is therefore not always accurate.
Moreover, applications exist in which the eyepiece/image plane setting is finer, for example in electron microscopy. In electron microscopy the operator can adjust the eyepiece/image plane setting visually. The operator can also make the setting using a technique known as the xe2x80x9cwobber focusing aidxe2x80x9d. The technique is described on pages 29 to 31 of xe2x80x9cThe Principles and Practice of Electron Microscopyxe2x80x9d by Ian M. Watt (Cambridge University Press). This technique consists in oscillating a luminous beam between two positions in relation to the lens of the image recording apparatus. This produces a double response of the object observed on the image plane until the system reaches optimal focusing.
However, these xe2x80x9cwobblingxe2x80x9d methods are difficult to implement as it is not easy to deflect the light source.
In astronomy, it is also useful to have fine setting, particularly when a CCD camera is used in combination with an astronomic eyepiece as described in the article xe2x80x9cUne Mission Haute Rxc3xa9solution au T60xe2x80x9d (A High Definition Assignment at T 60) by J. Dijon et al., published in Pulsar magazine No. 707, March-April 1995. The article describes a method that enables a relatively sensitive eyepiece/image plane setting to be achieved. The method consists in selecting a single star, positioning a two-holed mask at the entry of the astronomic eyepiece and verifying the number of patches of light that appear on the CCD image recording apparatus. If the CCD camera is not correctly focused in relation to the eyepiece, two patches appear on the camera. If, however, the focusing is correct only a single patch appears on the CCD image recording apparatus. By displacing the projection of the star over different areas of the CCD camera, the alignment of the eyepiece/CCD camera can be adjusted, in other words it is possible to adjust the alignment between the eyepiece and the image plane.
However, this method can only be used for an object located at infinity, as is the case in astronomy.
Other optical methods enable the orthogonality of the image plane with the optical axis to be adjusted. One method consists in using an autocollimation eyepiece that is placed on the entry of the eyepiece. In this example, the image plane must provide a reflection for this kind of setting. Moreover, difficulties may arise when other reflective planes are present on the trajectory between the eyepiece and the image plane, as is the case in a CCD camera being used through a glass window.
A method of this kind therefore proves difficult to put into operation. Also, this method only provides the setting required for surface evenness, it does not provide the setting for focusing required to align the system.
The aim of the invention is to overcome the drawbacks of the techniques described above. In order to do this, the invention provides a method for aligning an object, of which an optimal quality shot is to be taken, with an image capture apparatus and a coupling eyepiece. This method consists in using a light source that is positioned in the place of the object and that emits luminous dots through an opaque mask, perforated with at least two holes, and through the coupling eyepiece in the direction of the capture apparatus. The method also consists in verifying whether the images of the luminous dots are split into two on the image plane of the capture apparatus.
More precisely, the invention relates to a method for aligning an object whose image is to be recorded with an image capture apparatus and a coupling eyepiece. This method is characterized in that it consists in:
defining an object plane
emitting at least three luminous dots in the object plane;
positioning an opaque mask perforated with at least two holes in the optical path;
effecting a plurality of image captures of the luminous dots, each image capture being made for a different alignment setting, using known parameter settings;
producing a list of distances between the images of the luminous dots on the image plane that is defined by the capture apparatus, said list being produced for each luminous dot and each alignment setting;
determining a development function of the distance between luminous dots for each luminous dot and each alignment setting; and
determining a particular setting for which the distances between the images of the luminous dots are minimal that is based on all the development functions for the various alignment settings.
Advantageously, the parameter settings comprise a translation movement parameter and two rotation parameters that move in a direction parallel to the optical axis and the coupling eyepiece.
The parameter settings can also include a fourth parameter that is relative to the overall translation movement of the image plane along the optical axis.
In one embodiment of the invention, the method consists in emitting three luminous dots and effecting at least four image capture operations of the luminous dots fore four different alignment settings.
In another embodiment of the invention, when the number of luminous dots is greater than 3, the method consists in pre-selecting those luminous dots that are to be included in the subsequent stages of the method of the invention.